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Branden Katona
and
Paul Markowski

Abstract

Storms crossing complex terrain can potentially encounter rapidly changing convective environments. However, our understanding of terrain-induced variability in convective storm environments remains limited. HRRR data are used to create climatologies of popular convective storm forecasting parameters for different wind regimes. Self-organizing maps (SOMs) are used to generate six different low-level wind regimes, characterized by different wind directions, for which popular instability and vertical wind shear parameters are averaged. The climatologies show that both instability and vertical wind shear are highly variable in regions of complex terrain, and that the spatial distributions of perturbations relative to the terrain are dependent on the low-level wind direction. Idealized simulations are used to investigate the origins of some of the perturbations seen in the SOM climatologies. The idealized simulations replicate many of the features in the SOM climatologies, which facilitates analysis of their dynamical origins. Terrain influences are greatest when winds are approximately perpendicular to the terrain. In such cases, a standing wave can develop in the lee, leading to an increase in low-level wind speed and a reduction in vertical wind shear with the valley lee of the plateau. Additionally, CAPE tends to be decreased and LCL heights are increased in the lee of the terrain where relative humidity within the boundary layer is locally decreased.

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Branden Katona
,
Paul Markowski
,
Curtis Alexander
, and
Stanley Benjamin

Abstract

Relatively little is known about how topography affects convective storms. The first step toward understanding these effects is to investigate how topography affects storm environments. Unfortunately, the effects of topography on convective environments are not easily observed directly. Instead, it is necessary to resort to using output from the High-Resolution Rapid Refresh (HRRR). The HRRR’s 3-km grid spacing can resolve some of the larger-scale topographic effects. Popular convective storm forecasting parameters obtained from the HRRR are averaged on convective days from February to September 2013–15. It is surmised that most of the day-to-day variability attributable to synoptic- and mesoscale meteorological influences is removed by averaging; the remaining horizontal heterogeneity in parameters related to instability and vertical wind shear is due to the hemispheric-scale meridional temperature and pressure gradient, and likely also topographic influences, especially where recurring longitudinal variations in instability, wind shear, etc. are found. Anomalies are sensitive to the ambient low-level wind direction (i.e., whether winds are locally blowing upslope or downslope), especially for parameters that depend on the low-level vertical shear. The statistical significance of local maxima and minima is demonstrated by comparing the amplitudes of the anomalies to bootstrapped estimates of the standard errors.

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Paul Markowski
,
Yvette Richardson
,
Matthew Kumjian
,
Alexandra Anderson-Frey
,
Giovanni Jimenez
,
Branden Katona
,
Alicia Klees
,
Robert Schrom
, and
Dana Tobin
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